The present invention relates to a technique for thermal transfer recording by which images are superposedly recorded with preferable conditions, such as less positional misregistration, and/or less hue misregistration and superior hue repeatability in color images. In particular, the present invention relates to a method and apparatus preferred to thermal transfer recording where an objective image is produced by first performing thermal transfer recording of images on an intermediate transfer recording medium, and then the medium is transferred to a transfer receiving material.
Conventionally, there have been proposed a variety of thermal transfer recording techniques for recording images, in which a thermal transfer sheet, having a substrate sheet on which a coloring material layer is disposed on one of the surfaces, and a transfer receiving material, on which a receptor layer is disposed, are pressed between a heating device such as a thermal head and a platen roll, and the heat-generating members of the heating device are selectively heated depending on image information. As a result, the coloring material contained by the coloring material layer of the thermal transfer sheet migrates to the transfer receiving material, realizing the thermal transfer recording. Of these heat fusion type thermal transfer and sublimation type thermal transfer methods have been most commonly used.
The heat fusion type thermal transfer method uses a thermal transfer sheet that has a substrate sheet, such as a plastic film, on which a heat fusible ink layer is coated as a coloring material layer, the ink layer being formed by dispersing coloring material such as pigment into heat fusible binder such as wax or resin. Onto this thermal transfer sheet, thermal energy according to image information is applied with such a heating device as a thermal head, thereby the coloring material being transferred together with the binder on such a transfer receiving material as a paper or plastic sheet. Images produced by this heat fusion type thermal transfer method have higher density and superior sharpness, and is preferable to recording of binary images consisting letters and/or drawings.
By contrast, the sublimation type thermal transfer method uses a thermal transfer sheet and a transfer receiving material. The former has such a substrate sheet as a plastic film, on which a dye layer is provided as a coloring material layer, the dye layer being formed by dissolving or dispersing a sublimation dye adopted as a coloring material into binder resin. The latter is formed by providing a receptor layer for a coloring material on a support medium, including paper or plastic sheets. Onto the thermal transfer sheet, thermal energy depending on image information is applied with a heating device such as a thermal head. As a result, only the coloring material contained in the coloring material layer of the thermal transfer sheet migrates to the receptor layer, realizing image recording.
In performing recording based on each transfer method, thermal transfer sheets for various colors including yellow(Y), magenta(M), cyan(C), and black(K) are used for recording of images in a superposition fashion onto a transfer receiving material, providing multi-color or full-color image recording.
Of these recording methods, it is particularly true that the sublimation type thermal transfer method requires the image-formed surface of its transfer receiving material to have a dyeing property for dye employed as coloring material. Hence, it was almost impossible to form images on transfer receiving materials that do not have a receptor layer having a dyeing property. For forming images by means of the sublimation type thermal transfer method on transfer receiving materials other than dedicated sheets of paper on which a receptor layer is formed beforehand, as shown in Japanese Patent Laid-open(KOKAI) publication No. 62-264994, for example, there has been proposed an image-forming technique that a receptor layer is transferred to a transfer receiving material from a receptor layer transfer sheet whose receptor layer is formed to be transferred peelably to a substrate sheet, and then a coloring material is transferred thereon from a thermal transfer sheet. However, when the method is adopted, the receptor layer formed on the transfer receiving materials is greatly affected by the characteristics of the surface thereof. In some cases, there occur such problems as voids of the receptor layer that arise in recessed portions of the surface of the transfer receiving material, and/or irregularities on the receptor layer affected by those on the surface of the transfer receiving material, both of them resulting in the formation of ununiform images. Therefore, it was necessary to select flat- or smooth-surface transfer receiving materials for producing satisfied objective images.
For preventing influences on image qualities due to surface irregularities and/or texture of a transfer receiving material, and for making it possible to form objective images on any transfer receiving material, another transfer technique is proposed, as in Japanese Patent Laid-open(KOKAI) Publication No.62-238791. In this technique, an intermediate transfer recording medium is first prepared that has a receptor layer peelable on a substrate sheet, images are transferred from a thermal transfer sheet to the receptor layer by means of the sublimation type thermal transfer method, and the intermediate transfer medium on which the images are formed is superposed on a transfer receiving material to be heated. This permits the image-formed receptor layer to be transferred, as a whole, to the transfer receiving material.
In order to accomodate as many intermediate transfer recording media as possible in a thermal transfer recording apparatus and to lower the material costs, thin films are favorably used as a substrate sheet thereof. However, owing to the fact that thin films are film-formed under orientation in their longitudinal and lateral directions in the production, there occurs thermal contraction in the films when exposed to higher-temperature heat after completion of manufacturing. The same event is caused when thin films are used as a substrate sheet of an intermediate transfer recording medium. Specifically, heat is applied by a thermal head in forming images in the receptor layer of an intermediate transfer recording medium, thus causing contraction therein. In particular, in recording color images, since an objective color image is produced by superposing each color image of yellow (Y), magenta (M), cyan (C), and the like one on another, inaccurate positioning for each color image brings about an extremely poor appearance. Still, if the contraction of the substrate sheet of the intermediate transfer recording medium is brought about every time when forming each color image on the medium, differences in sizes of color images are generated, thereby making it impossible to accomplish an accurate superposition. The contraction or enlargement in the lengthwise direction (i.e., the conveying direction for conveying an intermediate transfer recording medium) can be suppressed to some extent by controlling a tension of conveying, but necessarily remains. In addition, since it is normal that tension is not applied in the widthwise direction (i.e., a direction perpendicular to the conveying direction of the medium), the contraction in the widthwise direction cannot be suppressed.
One example is such that in the case of forming a color image in three colors of Y, M and C on an intermediate transfer recording medium using a polyethylene terephthalate film having a thickness of 10 .mu.m as a substrate, a contraction of approximately 0.6 to 1.0 mm over the lateral width of 180 mm was generated. This contraction amount agrees with a misregistration of approximately 8 to 13 dots in a picture element density of 12 dots/mm, and this amount of misregistration could not be ignored for producing a superior sharpness and an accurate color repeatability of images.